1. Field of the Invention
The present invention relates to a developer used for an image forming apparatus such as a copying machine or printer, and a process cartridge and an image forming apparatus used therefor.
2. Description of the Related Art
A number of electrophotographic processes are known in the art. In the electrophotographic process, a fixed image is formed through a plurality of steps comprising electrically forming a latent image by various methods on a photoreceptor utilizing a photoconductive material, developing the latent image using a toner, transferring the toner latent image on the photoreceptor onto a transfer object such as a sheet of paper with or without interposition of an intermediate transfer apparatus, and fixing the latent image by heating, compressing or heating with compression, or with a vapor of a solvent. If necessary, toner remaining on the photoreceptor is cleaned off by various methods, and the aforementioned plural steps are repeated. Printers and copying machines utilizing the electrophotographic process are widely used, and accordingly, requirements with respect to performance and image quality are become more strict year by year.
Developing methods for the electrophotographic process are divided into one-component developing methods and two-component developing methods. While the two-component developing method has widely been used for development since it is advantageous for high speed processing, it involves disadvantages such as deterioration of the developer due to adhesion of the toner on the surface of a carrier and a large size of the developing device since the mixing ratio of the toner to the carrier should be kept constant so that the toner concentration in the developer does not decrease due to only the toner being consumed in this method. Consequently, the cost of controlling the toner density becomes high. On the other hand, since the one-component developing method is advantageous in that the device is compact and the cost may be reduced without producing the above-mentioned defects, the device is prevalent in small offices and in the field of personal users.
The one-component developing method is roughly divided into a non-magnetic one-component developing method and magnetic one-component developing method. The former is suitable for color printing because the toner does not contain magnetic powders. On the other hand, the magnetic one-component developing method is frequently used in a monochromatic electrostatic copying method since the toner can be retained on the toner bearing body using a magnetic force of the magnetic powder contained in the toner, and from the viewpoints of good conveying ability of the toner and easily inhibition of fogging of the toner at non-image portions.
The one-component developing method is more suitable for making the apparatus compact as compared with the two-component developing method. However, while so-called long life is urgently required for increasing the number of printing sheets (the number of copying sheets) per process cartridge in addition to the requirement for more compactness in recent years, a space for accommodating the developer is further reduced by making the process cartridge small. Consequently, the number of printing or copying sheets printed or copied before the toner in the cartridge has been depleted is reduced. Therefore, various methods have been devised for satisfying requirements of both small size and long life.
Image forming apparatuses, to which process cartridges are detachably attached to main bodies thereof, are conventionally well-known. A conventional process cartridge of a cartridge wherein a cleaning device and a developing device are disposed in a positional relationship of one above the other with the scanning light path therebetween is known.
Particularly, for the sake of reducing the width of a machine, a recording medium is not discharged to the side of the machine, but preferably the recording medium path is arranged to be substantially vertical from a transferring step to a fixing step, to discharge the recording medium at an upper portion of the machine. However, the developing device is disposed under the scanning light path in an image forming apparatus as described above. Thus, since the developing device is disposed under the scanning light path, there is a need to reduce the size of the developing device in order to reduce the height of the apparatus. The developing device contains a developer storage portion for storing developer, so that the storage portion for the developer must be reduced, and it has been difficult to reduce the size of the apparatus while maintaining the developer storage capacity of the developer storage portion.